Transition Metal Nitride As Photocatalyst for Visible Light Driven Hydrogen Production

Photoelectrochemical splitting of water is one of the cleanest ways of producing hydrogen.  However, the efficiency of this process is less than 1% with the currently used materials.  Improvement in the electrode performance is limited by either chemical instability or poor light absorption properties of these materials.  Although transition metal oxides are relatively stable during gas evolution, they have wide band gaps and thus absorb only UV part of the solar spectrum.  Hence there is an intense search for newer materials with improved chemically stability and optical properties.  In this work, we performed post synthesis nitridation of oxide nanowire arrays to form a completely new nitrided phase that has a lower band gap.  The photoelectrochemical properties of these nanowire electrodes were characterized by UV-Vis, impedance, and photocurrent spectroscopy.  The results are discussed in terms of their water splitting efficiency.

Tungsten oxide nanowire arrays were synthesized by hot filament CVD on both FTO and quartz substrates.  Figure 1 shows the SEM image of WO₃ array on FTO substrates.  Post synthesis nitridation in ammonia resulted in the complete phase transformation to W₂N.  X-ray diffraction and transmission electron microscopy confirmed their high degree of crystallinity.

Figure 2 shows the Tauc plot obtained from UV-Vis spectroscopy.  Oxidized WO₃ has a band gap of 3 eV.  On the other hand nitridation leads to a reduction in the band gap from 3 eV to 2.5 eV.  The photoelectrochemical measurements were done in a three electrode configuration.  Figure 3 shows a representative I-V characteristics of WO₃ in dark and under simulated AM 1.5 solar light.  The electrodes were further characterized by impedance, and photocurrent spectroscopy. The results are discussed in terms of their water splitting efficiency.